I am trying to send out a mail merge MacBook word and outlook (outlook default) and getting failure code: c355

Failure code c355 is not documented in the provided context, but the context does cover common causes when Outlook for Mac cannot send messages and when Word mail merge to email fails.

Based on that information, use the following checks and workarounds:

1. Confirm Outlook for Mac can send a normal email
   • In Outlook for Mac, send a simple test message (no mail merge) to a known-good address.
   • If this fails, troubleshoot Outlook first:
     • Make sure the Mac has Internet access.
     • In Outlook, verify Outlook > Work Offline is not selected.
     • Check that the mailbox is not full using the mailbox storage page referenced in the Outlook for Mac article.
2. Verify account settings and SMTP authentication
   • In Outlook for Mac, go to Tools > Accounts.
   • Select the account used for mail merge.
   • Confirm the email address, username, and password are correct.
   • Under Outgoing server, if using POP/IMAP:
     • Select More Options and ensure the correct authentication type is set and credentials are entered, as some providers require SMTP authentication.
3. Check for firewall / network / proxy issues
   • If using a corporate or ISP network, a firewall may block the outgoing SMTP server.
   • Ask the ISP or network admin if an alternative SMTP server or port is required and update it under Tools > Accounts > Outgoing server.
   • If the network requires a proxy, ensure the mail proxy is configured in macOS System Settings as described in the Outlook for Mac article.
4. Confirm Outlook is the default MAPI-compatible mail client
   • For Word mail merge on macOS, a MAPI-compatible email program such as Outlook must be installed and set as default.
   • Ensure Outlook (not a non‑MAPI client) is the default mail app in macOS and that the same account is signed in to Word and Outlook.
5. Re-run the mail merge with these prerequisites
   • In Word on Mac:
     • Go to Mailings > Start Mail Merge > Email Messages.
     • Use Select Recipients to link the data source that contains a valid email column.
     • Insert merge fields via Mailings > Insert Merge Field.
     • Use Preview Results to confirm each record looks correct.
   • When ready, choose Mailings > Finish & Merge > Merge to E-Mail.
     • Select the To field (email column), subject, and format (HTML/text/attachment).
     • Select Mail Merge To Outbox.
   • If Merge to E-Mail is unavailable, set the default email program first as indicated in the article.
6. If messages still fail delivery
   • If Outlook shows non-delivery reports or “delivery failed” messages, review the returned message details (for example, as described for Outlook.com and postmaster messages in the context) to see if the issue is with the recipient server, invalid addresses, or message size.
   • Reduce the number of recipients per batch and confirm all recipient addresses are valid.
7. If none of the above resolves the issue
   • Use the Outlook.com / Outlook support entry points in the context (Help menu or the support links) to contact support, providing the full error text and the fact that the failure occurs during Word mail merge to Outlook on macOS.

References:

• Use mail merge in Word to send bulk email messages
• I can't send or receive messages with Outlook for Mac
• Sent email in Outlook.com comes back "delivery failed"
• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com
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